However, the required pilot overhead in [1]}, [2]} is huge. This is mainly caused by the fact that the number of unknown channel coefficients (e.g., \(64 \times 256\)  with 64 antennas at the BS, 256 elements at the RIS and single antenna at the user) in the RIS assisted communication system is much larger than that of unknown channel coefficients (e.g., \(64 \times 1\)  with 64 antennas at the BS and a single antenna at the user) in the conventional communication system without the RIS. The huge pilot overhead will significantly decrease the effective capacity improvement. Thus, it is essential to develop overhead-reduced channel estimation schemes for the RIS assisted system. In the next Section , we will introduce three typical types of overhead-reduced channel estimation solutions.
